The present invention relates to the art of processing information within magnetic fields. It finds particular application in conjunction with amplifiers for radio frequency pick-up coils in magnetic resonance imaging apparatus and will be described with particular reference thereto. However, it is to be appreciated that the invention will also find application in conjunction with packaging for other circuitry positioned within the magnetic field of magnetic resonance imaging and spectroscopy apparatus and the like.
Conventionally, patient encircling coils generate a static magnetic field through a patient examination region within their bore. Gradient magnetic field coils and RF coils are typically positioned concentrically within the static magnetic field coils, but outside the central examination region. The gradient magnetic field and radio frequency coils are controlled to generate corresponding gradient magnetic field and RF pulses for various magnetic resonance sequences, as is known in the art. Typically, the radio frequency coil can also be utilized to receive the relatively weak (as compared to the RF pulses of the magnetic resonance excitation sequence) magnetic resonance signals emanating from the subject as a result of the magnetic resonance sequence. Signals received by the radio frequency coil are conveyed by leads to an amplifier disposed sufficiently remote from the coils that it is shielded from the magnetic fields.
For some imaging procedures, a surface radio frequency coil is positioned within the examination region, typically firmly against a surface of the subject. This places the coil for receiving the magnetic resonance signals much closer to assist in receiving weaker signals, to limit the region of the subject from which magnetic resonance signals are received, and the like. Typically, the signals from the surface coils are conveyed over non-ferrous conductor cables to an amplifier located externally of the static magnetic field, typically a distance of a meter or more from the patient. The lead, to a certain extent, function as an antenna for picking up unwanted signals and noise which the amplifier amplifies along with the relatively weak magnetic resonance signals.
Placing the amplifier on the whole body radio frequency coil or surface coil could raise the amplitude of the magnetic resonance signal several orders of magnitude above the noise picked up on the leads. However, conventionally available circuit components include ferrous materials which would distort the magnetic field and be subject to torques exerted by the magnetic fields. Because the whole body radio frequency coils and the surface coils are positioned very close to the examined region of interest, any ferrous components can cause unacceptably large distortions in the static magnetic field and the resultant image.
Readily available circuit components, such as diodes, transistors, microprocessors, and the like include steel in their packaging. More specifically, the lead wires and connectors are typically fabricated of steel. High quality components with "gold" leads actually have gold plated steel leads.
Having a component developed which includes no steel or nickel in its packaging is a time consuming and expensive undertaking. Many component companies are unwilling to invest the time and resources necessary to develop non-ferrous packaging for the components. Those suppliers which will, require long lead times, typically on the order of 3-9 months. Moreover, the components which are supplied are several hundred times more expensive than their steel packaged counterparts.
In high volume consumer electronics applications, where small size is very important, chip and wire hybrid, chip on board, tape automated bonding, flip chip, and other technologies have been utilized. In these technologies, the active and other components of a given circuit are adhered directly to a circuit board or substrate without being encased in a steel or other housing. Appropriate whiskers or other electrical interconnections are provided and the entire package is encapsulated in epoxy. Due to developmental costs, the use of these circuits is limited to less cost-sensitive applications. Even in high volume applications, these technologies are rarely utilized unless small size is important. Due to the low volume of magnetic resonance imagers and surface coils and because each different surface coil model would require a different amplifier circuit, these technologies have been considered inappropriate for magnetic resonance scanners.
Moreover, the circuit boards used in some of these technologies such as chip-on-board technology, like other circuit boards, include an epoxy fiberglass or polyamide substrate that is laminated with copper which is etched away to form appropriate circuit leads. A nickel flash is applied to the copper followed by gold plating. Nickel, however, is sufficiently ferro-magnetic to cause significant distortions in the static magnetic field in the region of interest of the subject.
Rather than standard circuit boards, wiring patterns can be drawn on ceramic substrate with gold ink and fired. This technique is generally used for prototyping or in less cost-sensitive applications. Thin film sputtering and etching can also be used to manufacture gold film on ceramic boards.
The present invention contemplates a new and improved technique which enables active components such as diodes, transistors, and integrated circuits to be mounted directly on surface coils and other components which are mounted within the examination region of a magnetic resonance apparatus.